Root/mm/ksm.c

1/*
2 * Memory merging support.
3 *
4 * This code enables dynamic sharing of identical pages found in different
5 * memory areas, even if they are not shared by fork()
6 *
7 * Copyright (C) 2008-2009 Red Hat, Inc.
8 * Authors:
9 * Izik Eidus
10 * Andrea Arcangeli
11 * Chris Wright
12 * Hugh Dickins
13 *
14 * This work is licensed under the terms of the GNU GPL, version 2.
15 */
16
17#include <linux/errno.h>
18#include <linux/mm.h>
19#include <linux/fs.h>
20#include <linux/mman.h>
21#include <linux/sched.h>
22#include <linux/rwsem.h>
23#include <linux/pagemap.h>
24#include <linux/rmap.h>
25#include <linux/spinlock.h>
26#include <linux/jhash.h>
27#include <linux/delay.h>
28#include <linux/kthread.h>
29#include <linux/wait.h>
30#include <linux/slab.h>
31#include <linux/rbtree.h>
32#include <linux/memory.h>
33#include <linux/mmu_notifier.h>
34#include <linux/swap.h>
35#include <linux/ksm.h>
36
37#include <asm/tlbflush.h>
38#include "internal.h"
39
40/*
41 * A few notes about the KSM scanning process,
42 * to make it easier to understand the data structures below:
43 *
44 * In order to reduce excessive scanning, KSM sorts the memory pages by their
45 * contents into a data structure that holds pointers to the pages' locations.
46 *
47 * Since the contents of the pages may change at any moment, KSM cannot just
48 * insert the pages into a normal sorted tree and expect it to find anything.
49 * Therefore KSM uses two data structures - the stable and the unstable tree.
50 *
51 * The stable tree holds pointers to all the merged pages (ksm pages), sorted
52 * by their contents. Because each such page is write-protected, searching on
53 * this tree is fully assured to be working (except when pages are unmapped),
54 * and therefore this tree is called the stable tree.
55 *
56 * In addition to the stable tree, KSM uses a second data structure called the
57 * unstable tree: this tree holds pointers to pages which have been found to
58 * be "unchanged for a period of time". The unstable tree sorts these pages
59 * by their contents, but since they are not write-protected, KSM cannot rely
60 * upon the unstable tree to work correctly - the unstable tree is liable to
61 * be corrupted as its contents are modified, and so it is called unstable.
62 *
63 * KSM solves this problem by several techniques:
64 *
65 * 1) The unstable tree is flushed every time KSM completes scanning all
66 * memory areas, and then the tree is rebuilt again from the beginning.
67 * 2) KSM will only insert into the unstable tree, pages whose hash value
68 * has not changed since the previous scan of all memory areas.
69 * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the
70 * colors of the nodes and not on their contents, assuring that even when
71 * the tree gets "corrupted" it won't get out of balance, so scanning time
72 * remains the same (also, searching and inserting nodes in an rbtree uses
73 * the same algorithm, so we have no overhead when we flush and rebuild).
74 * 4) KSM never flushes the stable tree, which means that even if it were to
75 * take 10 attempts to find a page in the unstable tree, once it is found,
76 * it is secured in the stable tree. (When we scan a new page, we first
77 * compare it against the stable tree, and then against the unstable tree.)
78 */
79
80/**
81 * struct mm_slot - ksm information per mm that is being scanned
82 * @link: link to the mm_slots hash list
83 * @mm_list: link into the mm_slots list, rooted in ksm_mm_head
84 * @rmap_list: head for this mm_slot's singly-linked list of rmap_items
85 * @mm: the mm that this information is valid for
86 */
87struct mm_slot {
88    struct hlist_node link;
89    struct list_head mm_list;
90    struct rmap_item *rmap_list;
91    struct mm_struct *mm;
92};
93
94/**
95 * struct ksm_scan - cursor for scanning
96 * @mm_slot: the current mm_slot we are scanning
97 * @address: the next address inside that to be scanned
98 * @rmap_list: link to the next rmap to be scanned in the rmap_list
99 * @seqnr: count of completed full scans (needed when removing unstable node)
100 *
101 * There is only the one ksm_scan instance of this cursor structure.
102 */
103struct ksm_scan {
104    struct mm_slot *mm_slot;
105    unsigned long address;
106    struct rmap_item **rmap_list;
107    unsigned long seqnr;
108};
109
110/**
111 * struct stable_node - node of the stable rbtree
112 * @node: rb node of this ksm page in the stable tree
113 * @hlist: hlist head of rmap_items using this ksm page
114 * @kpfn: page frame number of this ksm page
115 */
116struct stable_node {
117    struct rb_node node;
118    struct hlist_head hlist;
119    unsigned long kpfn;
120};
121
122/**
123 * struct rmap_item - reverse mapping item for virtual addresses
124 * @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
125 * @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
126 * @mm: the memory structure this rmap_item is pointing into
127 * @address: the virtual address this rmap_item tracks (+ flags in low bits)
128 * @oldchecksum: previous checksum of the page at that virtual address
129 * @node: rb node of this rmap_item in the unstable tree
130 * @head: pointer to stable_node heading this list in the stable tree
131 * @hlist: link into hlist of rmap_items hanging off that stable_node
132 */
133struct rmap_item {
134    struct rmap_item *rmap_list;
135    struct anon_vma *anon_vma; /* when stable */
136    struct mm_struct *mm;
137    unsigned long address; /* + low bits used for flags below */
138    unsigned int oldchecksum; /* when unstable */
139    union {
140        struct rb_node node; /* when node of unstable tree */
141        struct { /* when listed from stable tree */
142            struct stable_node *head;
143            struct hlist_node hlist;
144        };
145    };
146};
147
148#define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
149#define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
150#define STABLE_FLAG 0x200 /* is listed from the stable tree */
151
152/* The stable and unstable tree heads */
153static struct rb_root root_stable_tree = RB_ROOT;
154static struct rb_root root_unstable_tree = RB_ROOT;
155
156#define MM_SLOTS_HASH_HEADS 1024
157static struct hlist_head *mm_slots_hash;
158
159static struct mm_slot ksm_mm_head = {
160    .mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list),
161};
162static struct ksm_scan ksm_scan = {
163    .mm_slot = &ksm_mm_head,
164};
165
166static struct kmem_cache *rmap_item_cache;
167static struct kmem_cache *stable_node_cache;
168static struct kmem_cache *mm_slot_cache;
169
170/* The number of nodes in the stable tree */
171static unsigned long ksm_pages_shared;
172
173/* The number of page slots additionally sharing those nodes */
174static unsigned long ksm_pages_sharing;
175
176/* The number of nodes in the unstable tree */
177static unsigned long ksm_pages_unshared;
178
179/* The number of rmap_items in use: to calculate pages_volatile */
180static unsigned long ksm_rmap_items;
181
182/* Number of pages ksmd should scan in one batch */
183static unsigned int ksm_thread_pages_to_scan = 100;
184
185/* Milliseconds ksmd should sleep between batches */
186static unsigned int ksm_thread_sleep_millisecs = 20;
187
188#define KSM_RUN_STOP 0
189#define KSM_RUN_MERGE 1
190#define KSM_RUN_UNMERGE 2
191static unsigned int ksm_run = KSM_RUN_STOP;
192
193static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
194static DEFINE_MUTEX(ksm_thread_mutex);
195static DEFINE_SPINLOCK(ksm_mmlist_lock);
196
197#define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
198        sizeof(struct __struct), __alignof__(struct __struct),\
199        (__flags), NULL)
200
201static int __init ksm_slab_init(void)
202{
203    rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
204    if (!rmap_item_cache)
205        goto out;
206
207    stable_node_cache = KSM_KMEM_CACHE(stable_node, 0);
208    if (!stable_node_cache)
209        goto out_free1;
210
211    mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
212    if (!mm_slot_cache)
213        goto out_free2;
214
215    return 0;
216
217out_free2:
218    kmem_cache_destroy(stable_node_cache);
219out_free1:
220    kmem_cache_destroy(rmap_item_cache);
221out:
222    return -ENOMEM;
223}
224
225static void __init ksm_slab_free(void)
226{
227    kmem_cache_destroy(mm_slot_cache);
228    kmem_cache_destroy(stable_node_cache);
229    kmem_cache_destroy(rmap_item_cache);
230    mm_slot_cache = NULL;
231}
232
233static inline struct rmap_item *alloc_rmap_item(void)
234{
235    struct rmap_item *rmap_item;
236
237    rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
238    if (rmap_item)
239        ksm_rmap_items++;
240    return rmap_item;
241}
242
243static inline void free_rmap_item(struct rmap_item *rmap_item)
244{
245    ksm_rmap_items--;
246    rmap_item->mm = NULL; /* debug safety */
247    kmem_cache_free(rmap_item_cache, rmap_item);
248}
249
250static inline struct stable_node *alloc_stable_node(void)
251{
252    return kmem_cache_alloc(stable_node_cache, GFP_KERNEL);
253}
254
255static inline void free_stable_node(struct stable_node *stable_node)
256{
257    kmem_cache_free(stable_node_cache, stable_node);
258}
259
260static inline struct mm_slot *alloc_mm_slot(void)
261{
262    if (!mm_slot_cache) /* initialization failed */
263        return NULL;
264    return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
265}
266
267static inline void free_mm_slot(struct mm_slot *mm_slot)
268{
269    kmem_cache_free(mm_slot_cache, mm_slot);
270}
271
272static int __init mm_slots_hash_init(void)
273{
274    mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head),
275                GFP_KERNEL);
276    if (!mm_slots_hash)
277        return -ENOMEM;
278    return 0;
279}
280
281static void __init mm_slots_hash_free(void)
282{
283    kfree(mm_slots_hash);
284}
285
286static struct mm_slot *get_mm_slot(struct mm_struct *mm)
287{
288    struct mm_slot *mm_slot;
289    struct hlist_head *bucket;
290    struct hlist_node *node;
291
292    bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
293                % MM_SLOTS_HASH_HEADS];
294    hlist_for_each_entry(mm_slot, node, bucket, link) {
295        if (mm == mm_slot->mm)
296            return mm_slot;
297    }
298    return NULL;
299}
300
301static void insert_to_mm_slots_hash(struct mm_struct *mm,
302                    struct mm_slot *mm_slot)
303{
304    struct hlist_head *bucket;
305
306    bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
307                % MM_SLOTS_HASH_HEADS];
308    mm_slot->mm = mm;
309    hlist_add_head(&mm_slot->link, bucket);
310}
311
312static inline int in_stable_tree(struct rmap_item *rmap_item)
313{
314    return rmap_item->address & STABLE_FLAG;
315}
316
317static void hold_anon_vma(struct rmap_item *rmap_item,
318              struct anon_vma *anon_vma)
319{
320    rmap_item->anon_vma = anon_vma;
321    atomic_inc(&anon_vma->external_refcount);
322}
323
324static void drop_anon_vma(struct rmap_item *rmap_item)
325{
326    struct anon_vma *anon_vma = rmap_item->anon_vma;
327
328    if (atomic_dec_and_lock(&anon_vma->external_refcount, &anon_vma->lock)) {
329        int empty = list_empty(&anon_vma->head);
330        spin_unlock(&anon_vma->lock);
331        if (empty)
332            anon_vma_free(anon_vma);
333    }
334}
335
336/*
337 * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
338 * page tables after it has passed through ksm_exit() - which, if necessary,
339 * takes mmap_sem briefly to serialize against them. ksm_exit() does not set
340 * a special flag: they can just back out as soon as mm_users goes to zero.
341 * ksm_test_exit() is used throughout to make this test for exit: in some
342 * places for correctness, in some places just to avoid unnecessary work.
343 */
344static inline bool ksm_test_exit(struct mm_struct *mm)
345{
346    return atomic_read(&mm->mm_users) == 0;
347}
348
349/*
350 * We use break_ksm to break COW on a ksm page: it's a stripped down
351 *
352 * if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
353 * put_page(page);
354 *
355 * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
356 * in case the application has unmapped and remapped mm,addr meanwhile.
357 * Could a ksm page appear anywhere else? Actually yes, in a VM_PFNMAP
358 * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
359 */
360static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
361{
362    struct page *page;
363    int ret = 0;
364
365    do {
366        cond_resched();
367        page = follow_page(vma, addr, FOLL_GET);
368        if (IS_ERR_OR_NULL(page))
369            break;
370        if (PageKsm(page))
371            ret = handle_mm_fault(vma->vm_mm, vma, addr,
372                            FAULT_FLAG_WRITE);
373        else
374            ret = VM_FAULT_WRITE;
375        put_page(page);
376    } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_OOM)));
377    /*
378     * We must loop because handle_mm_fault() may back out if there's
379     * any difficulty e.g. if pte accessed bit gets updated concurrently.
380     *
381     * VM_FAULT_WRITE is what we have been hoping for: it indicates that
382     * COW has been broken, even if the vma does not permit VM_WRITE;
383     * but note that a concurrent fault might break PageKsm for us.
384     *
385     * VM_FAULT_SIGBUS could occur if we race with truncation of the
386     * backing file, which also invalidates anonymous pages: that's
387     * okay, that truncation will have unmapped the PageKsm for us.
388     *
389     * VM_FAULT_OOM: at the time of writing (late July 2009), setting
390     * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
391     * current task has TIF_MEMDIE set, and will be OOM killed on return
392     * to user; and ksmd, having no mm, would never be chosen for that.
393     *
394     * But if the mm is in a limited mem_cgroup, then the fault may fail
395     * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
396     * even ksmd can fail in this way - though it's usually breaking ksm
397     * just to undo a merge it made a moment before, so unlikely to oom.
398     *
399     * That's a pity: we might therefore have more kernel pages allocated
400     * than we're counting as nodes in the stable tree; but ksm_do_scan
401     * will retry to break_cow on each pass, so should recover the page
402     * in due course. The important thing is to not let VM_MERGEABLE
403     * be cleared while any such pages might remain in the area.
404     */
405    return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
406}
407
408static void break_cow(struct rmap_item *rmap_item)
409{
410    struct mm_struct *mm = rmap_item->mm;
411    unsigned long addr = rmap_item->address;
412    struct vm_area_struct *vma;
413
414    /*
415     * It is not an accident that whenever we want to break COW
416     * to undo, we also need to drop a reference to the anon_vma.
417     */
418    drop_anon_vma(rmap_item);
419
420    down_read(&mm->mmap_sem);
421    if (ksm_test_exit(mm))
422        goto out;
423    vma = find_vma(mm, addr);
424    if (!vma || vma->vm_start > addr)
425        goto out;
426    if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
427        goto out;
428    break_ksm(vma, addr);
429out:
430    up_read(&mm->mmap_sem);
431}
432
433static struct page *get_mergeable_page(struct rmap_item *rmap_item)
434{
435    struct mm_struct *mm = rmap_item->mm;
436    unsigned long addr = rmap_item->address;
437    struct vm_area_struct *vma;
438    struct page *page;
439
440    down_read(&mm->mmap_sem);
441    if (ksm_test_exit(mm))
442        goto out;
443    vma = find_vma(mm, addr);
444    if (!vma || vma->vm_start > addr)
445        goto out;
446    if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
447        goto out;
448
449    page = follow_page(vma, addr, FOLL_GET);
450    if (IS_ERR_OR_NULL(page))
451        goto out;
452    if (PageAnon(page)) {
453        flush_anon_page(vma, page, addr);
454        flush_dcache_page(page);
455    } else {
456        put_page(page);
457out: page = NULL;
458    }
459    up_read(&mm->mmap_sem);
460    return page;
461}
462
463static void remove_node_from_stable_tree(struct stable_node *stable_node)
464{
465    struct rmap_item *rmap_item;
466    struct hlist_node *hlist;
467
468    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
469        if (rmap_item->hlist.next)
470            ksm_pages_sharing--;
471        else
472            ksm_pages_shared--;
473        drop_anon_vma(rmap_item);
474        rmap_item->address &= PAGE_MASK;
475        cond_resched();
476    }
477
478    rb_erase(&stable_node->node, &root_stable_tree);
479    free_stable_node(stable_node);
480}
481
482/*
483 * get_ksm_page: checks if the page indicated by the stable node
484 * is still its ksm page, despite having held no reference to it.
485 * In which case we can trust the content of the page, and it
486 * returns the gotten page; but if the page has now been zapped,
487 * remove the stale node from the stable tree and return NULL.
488 *
489 * You would expect the stable_node to hold a reference to the ksm page.
490 * But if it increments the page's count, swapping out has to wait for
491 * ksmd to come around again before it can free the page, which may take
492 * seconds or even minutes: much too unresponsive. So instead we use a
493 * "keyhole reference": access to the ksm page from the stable node peeps
494 * out through its keyhole to see if that page still holds the right key,
495 * pointing back to this stable node. This relies on freeing a PageAnon
496 * page to reset its page->mapping to NULL, and relies on no other use of
497 * a page to put something that might look like our key in page->mapping.
498 *
499 * include/linux/pagemap.h page_cache_get_speculative() is a good reference,
500 * but this is different - made simpler by ksm_thread_mutex being held, but
501 * interesting for assuming that no other use of the struct page could ever
502 * put our expected_mapping into page->mapping (or a field of the union which
503 * coincides with page->mapping). The RCU calls are not for KSM at all, but
504 * to keep the page_count protocol described with page_cache_get_speculative.
505 *
506 * Note: it is possible that get_ksm_page() will return NULL one moment,
507 * then page the next, if the page is in between page_freeze_refs() and
508 * page_unfreeze_refs(): this shouldn't be a problem anywhere, the page
509 * is on its way to being freed; but it is an anomaly to bear in mind.
510 */
511static struct page *get_ksm_page(struct stable_node *stable_node)
512{
513    struct page *page;
514    void *expected_mapping;
515
516    page = pfn_to_page(stable_node->kpfn);
517    expected_mapping = (void *)stable_node +
518                (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
519    rcu_read_lock();
520    if (page->mapping != expected_mapping)
521        goto stale;
522    if (!get_page_unless_zero(page))
523        goto stale;
524    if (page->mapping != expected_mapping) {
525        put_page(page);
526        goto stale;
527    }
528    rcu_read_unlock();
529    return page;
530stale:
531    rcu_read_unlock();
532    remove_node_from_stable_tree(stable_node);
533    return NULL;
534}
535
536/*
537 * Removing rmap_item from stable or unstable tree.
538 * This function will clean the information from the stable/unstable tree.
539 */
540static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
541{
542    if (rmap_item->address & STABLE_FLAG) {
543        struct stable_node *stable_node;
544        struct page *page;
545
546        stable_node = rmap_item->head;
547        page = get_ksm_page(stable_node);
548        if (!page)
549            goto out;
550
551        lock_page(page);
552        hlist_del(&rmap_item->hlist);
553        unlock_page(page);
554        put_page(page);
555
556        if (stable_node->hlist.first)
557            ksm_pages_sharing--;
558        else
559            ksm_pages_shared--;
560
561        drop_anon_vma(rmap_item);
562        rmap_item->address &= PAGE_MASK;
563
564    } else if (rmap_item->address & UNSTABLE_FLAG) {
565        unsigned char age;
566        /*
567         * Usually ksmd can and must skip the rb_erase, because
568         * root_unstable_tree was already reset to RB_ROOT.
569         * But be careful when an mm is exiting: do the rb_erase
570         * if this rmap_item was inserted by this scan, rather
571         * than left over from before.
572         */
573        age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
574        BUG_ON(age > 1);
575        if (!age)
576            rb_erase(&rmap_item->node, &root_unstable_tree);
577
578        ksm_pages_unshared--;
579        rmap_item->address &= PAGE_MASK;
580    }
581out:
582    cond_resched(); /* we're called from many long loops */
583}
584
585static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
586                       struct rmap_item **rmap_list)
587{
588    while (*rmap_list) {
589        struct rmap_item *rmap_item = *rmap_list;
590        *rmap_list = rmap_item->rmap_list;
591        remove_rmap_item_from_tree(rmap_item);
592        free_rmap_item(rmap_item);
593    }
594}
595
596/*
597 * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather
598 * than check every pte of a given vma, the locking doesn't quite work for
599 * that - an rmap_item is assigned to the stable tree after inserting ksm
600 * page and upping mmap_sem. Nor does it fit with the way we skip dup'ing
601 * rmap_items from parent to child at fork time (so as not to waste time
602 * if exit comes before the next scan reaches it).
603 *
604 * Similarly, although we'd like to remove rmap_items (so updating counts
605 * and freeing memory) when unmerging an area, it's easier to leave that
606 * to the next pass of ksmd - consider, for example, how ksmd might be
607 * in cmp_and_merge_page on one of the rmap_items we would be removing.
608 */
609static int unmerge_ksm_pages(struct vm_area_struct *vma,
610                 unsigned long start, unsigned long end)
611{
612    unsigned long addr;
613    int err = 0;
614
615    for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
616        if (ksm_test_exit(vma->vm_mm))
617            break;
618        if (signal_pending(current))
619            err = -ERESTARTSYS;
620        else
621            err = break_ksm(vma, addr);
622    }
623    return err;
624}
625
626#ifdef CONFIG_SYSFS
627/*
628 * Only called through the sysfs control interface:
629 */
630static int unmerge_and_remove_all_rmap_items(void)
631{
632    struct mm_slot *mm_slot;
633    struct mm_struct *mm;
634    struct vm_area_struct *vma;
635    int err = 0;
636
637    spin_lock(&ksm_mmlist_lock);
638    ksm_scan.mm_slot = list_entry(ksm_mm_head.mm_list.next,
639                        struct mm_slot, mm_list);
640    spin_unlock(&ksm_mmlist_lock);
641
642    for (mm_slot = ksm_scan.mm_slot;
643            mm_slot != &ksm_mm_head; mm_slot = ksm_scan.mm_slot) {
644        mm = mm_slot->mm;
645        down_read(&mm->mmap_sem);
646        for (vma = mm->mmap; vma; vma = vma->vm_next) {
647            if (ksm_test_exit(mm))
648                break;
649            if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
650                continue;
651            err = unmerge_ksm_pages(vma,
652                        vma->vm_start, vma->vm_end);
653            if (err)
654                goto error;
655        }
656
657        remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list);
658
659        spin_lock(&ksm_mmlist_lock);
660        ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
661                        struct mm_slot, mm_list);
662        if (ksm_test_exit(mm)) {
663            hlist_del(&mm_slot->link);
664            list_del(&mm_slot->mm_list);
665            spin_unlock(&ksm_mmlist_lock);
666
667            free_mm_slot(mm_slot);
668            clear_bit(MMF_VM_MERGEABLE, &mm->flags);
669            up_read(&mm->mmap_sem);
670            mmdrop(mm);
671        } else {
672            spin_unlock(&ksm_mmlist_lock);
673            up_read(&mm->mmap_sem);
674        }
675    }
676
677    ksm_scan.seqnr = 0;
678    return 0;
679
680error:
681    up_read(&mm->mmap_sem);
682    spin_lock(&ksm_mmlist_lock);
683    ksm_scan.mm_slot = &ksm_mm_head;
684    spin_unlock(&ksm_mmlist_lock);
685    return err;
686}
687#endif /* CONFIG_SYSFS */
688
689static u32 calc_checksum(struct page *page)
690{
691    u32 checksum;
692    void *addr = kmap_atomic(page, KM_USER0);
693    checksum = jhash2(addr, PAGE_SIZE / 4, 17);
694    kunmap_atomic(addr, KM_USER0);
695    return checksum;
696}
697
698static int memcmp_pages(struct page *page1, struct page *page2)
699{
700    char *addr1, *addr2;
701    int ret;
702
703    addr1 = kmap_atomic(page1, KM_USER0);
704    addr2 = kmap_atomic(page2, KM_USER1);
705    ret = memcmp(addr1, addr2, PAGE_SIZE);
706    kunmap_atomic(addr2, KM_USER1);
707    kunmap_atomic(addr1, KM_USER0);
708    return ret;
709}
710
711static inline int pages_identical(struct page *page1, struct page *page2)
712{
713    return !memcmp_pages(page1, page2);
714}
715
716static int write_protect_page(struct vm_area_struct *vma, struct page *page,
717                  pte_t *orig_pte)
718{
719    struct mm_struct *mm = vma->vm_mm;
720    unsigned long addr;
721    pte_t *ptep;
722    spinlock_t *ptl;
723    int swapped;
724    int err = -EFAULT;
725
726    addr = page_address_in_vma(page, vma);
727    if (addr == -EFAULT)
728        goto out;
729
730    ptep = page_check_address(page, mm, addr, &ptl, 0);
731    if (!ptep)
732        goto out;
733
734    if (pte_write(*ptep)) {
735        pte_t entry;
736
737        swapped = PageSwapCache(page);
738        flush_cache_page(vma, addr, page_to_pfn(page));
739        /*
740         * Ok this is tricky, when get_user_pages_fast() run it doesnt
741         * take any lock, therefore the check that we are going to make
742         * with the pagecount against the mapcount is racey and
743         * O_DIRECT can happen right after the check.
744         * So we clear the pte and flush the tlb before the check
745         * this assure us that no O_DIRECT can happen after the check
746         * or in the middle of the check.
747         */
748        entry = ptep_clear_flush(vma, addr, ptep);
749        /*
750         * Check that no O_DIRECT or similar I/O is in progress on the
751         * page
752         */
753        if (page_mapcount(page) + 1 + swapped != page_count(page)) {
754            set_pte_at(mm, addr, ptep, entry);
755            goto out_unlock;
756        }
757        entry = pte_wrprotect(entry);
758        set_pte_at_notify(mm, addr, ptep, entry);
759    }
760    *orig_pte = *ptep;
761    err = 0;
762
763out_unlock:
764    pte_unmap_unlock(ptep, ptl);
765out:
766    return err;
767}
768
769/**
770 * replace_page - replace page in vma by new ksm page
771 * @vma: vma that holds the pte pointing to page
772 * @page: the page we are replacing by kpage
773 * @kpage: the ksm page we replace page by
774 * @orig_pte: the original value of the pte
775 *
776 * Returns 0 on success, -EFAULT on failure.
777 */
778static int replace_page(struct vm_area_struct *vma, struct page *page,
779            struct page *kpage, pte_t orig_pte)
780{
781    struct mm_struct *mm = vma->vm_mm;
782    pgd_t *pgd;
783    pud_t *pud;
784    pmd_t *pmd;
785    pte_t *ptep;
786    spinlock_t *ptl;
787    unsigned long addr;
788    int err = -EFAULT;
789
790    addr = page_address_in_vma(page, vma);
791    if (addr == -EFAULT)
792        goto out;
793
794    pgd = pgd_offset(mm, addr);
795    if (!pgd_present(*pgd))
796        goto out;
797
798    pud = pud_offset(pgd, addr);
799    if (!pud_present(*pud))
800        goto out;
801
802    pmd = pmd_offset(pud, addr);
803    if (!pmd_present(*pmd))
804        goto out;
805
806    ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
807    if (!pte_same(*ptep, orig_pte)) {
808        pte_unmap_unlock(ptep, ptl);
809        goto out;
810    }
811
812    get_page(kpage);
813    page_add_anon_rmap(kpage, vma, addr);
814
815    flush_cache_page(vma, addr, pte_pfn(*ptep));
816    ptep_clear_flush(vma, addr, ptep);
817    set_pte_at_notify(mm, addr, ptep, mk_pte(kpage, vma->vm_page_prot));
818
819    page_remove_rmap(page);
820    put_page(page);
821
822    pte_unmap_unlock(ptep, ptl);
823    err = 0;
824out:
825    return err;
826}
827
828/*
829 * try_to_merge_one_page - take two pages and merge them into one
830 * @vma: the vma that holds the pte pointing to page
831 * @page: the PageAnon page that we want to replace with kpage
832 * @kpage: the PageKsm page that we want to map instead of page,
833 * or NULL the first time when we want to use page as kpage.
834 *
835 * This function returns 0 if the pages were merged, -EFAULT otherwise.
836 */
837static int try_to_merge_one_page(struct vm_area_struct *vma,
838                 struct page *page, struct page *kpage)
839{
840    pte_t orig_pte = __pte(0);
841    int err = -EFAULT;
842
843    if (page == kpage) /* ksm page forked */
844        return 0;
845
846    if (!(vma->vm_flags & VM_MERGEABLE))
847        goto out;
848    if (!PageAnon(page))
849        goto out;
850
851    /*
852     * We need the page lock to read a stable PageSwapCache in
853     * write_protect_page(). We use trylock_page() instead of
854     * lock_page() because we don't want to wait here - we
855     * prefer to continue scanning and merging different pages,
856     * then come back to this page when it is unlocked.
857     */
858    if (!trylock_page(page))
859        goto out;
860    /*
861     * If this anonymous page is mapped only here, its pte may need
862     * to be write-protected. If it's mapped elsewhere, all of its
863     * ptes are necessarily already write-protected. But in either
864     * case, we need to lock and check page_count is not raised.
865     */
866    if (write_protect_page(vma, page, &orig_pte) == 0) {
867        if (!kpage) {
868            /*
869             * While we hold page lock, upgrade page from
870             * PageAnon+anon_vma to PageKsm+NULL stable_node:
871             * stable_tree_insert() will update stable_node.
872             */
873            set_page_stable_node(page, NULL);
874            mark_page_accessed(page);
875            err = 0;
876        } else if (pages_identical(page, kpage))
877            err = replace_page(vma, page, kpage, orig_pte);
878    }
879
880    if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
881        munlock_vma_page(page);
882        if (!PageMlocked(kpage)) {
883            unlock_page(page);
884            lock_page(kpage);
885            mlock_vma_page(kpage);
886            page = kpage; /* for final unlock */
887        }
888    }
889
890    unlock_page(page);
891out:
892    return err;
893}
894
895/*
896 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
897 * but no new kernel page is allocated: kpage must already be a ksm page.
898 *
899 * This function returns 0 if the pages were merged, -EFAULT otherwise.
900 */
901static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
902                      struct page *page, struct page *kpage)
903{
904    struct mm_struct *mm = rmap_item->mm;
905    struct vm_area_struct *vma;
906    int err = -EFAULT;
907
908    down_read(&mm->mmap_sem);
909    if (ksm_test_exit(mm))
910        goto out;
911    vma = find_vma(mm, rmap_item->address);
912    if (!vma || vma->vm_start > rmap_item->address)
913        goto out;
914
915    err = try_to_merge_one_page(vma, page, kpage);
916    if (err)
917        goto out;
918
919    /* Must get reference to anon_vma while still holding mmap_sem */
920    hold_anon_vma(rmap_item, vma->anon_vma);
921out:
922    up_read(&mm->mmap_sem);
923    return err;
924}
925
926/*
927 * try_to_merge_two_pages - take two identical pages and prepare them
928 * to be merged into one page.
929 *
930 * This function returns the kpage if we successfully merged two identical
931 * pages into one ksm page, NULL otherwise.
932 *
933 * Note that this function upgrades page to ksm page: if one of the pages
934 * is already a ksm page, try_to_merge_with_ksm_page should be used.
935 */
936static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
937                       struct page *page,
938                       struct rmap_item *tree_rmap_item,
939                       struct page *tree_page)
940{
941    int err;
942
943    err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
944    if (!err) {
945        err = try_to_merge_with_ksm_page(tree_rmap_item,
946                            tree_page, page);
947        /*
948         * If that fails, we have a ksm page with only one pte
949         * pointing to it: so break it.
950         */
951        if (err)
952            break_cow(rmap_item);
953    }
954    return err ? NULL : page;
955}
956
957/*
958 * stable_tree_search - search for page inside the stable tree
959 *
960 * This function checks if there is a page inside the stable tree
961 * with identical content to the page that we are scanning right now.
962 *
963 * This function returns the stable tree node of identical content if found,
964 * NULL otherwise.
965 */
966static struct page *stable_tree_search(struct page *page)
967{
968    struct rb_node *node = root_stable_tree.rb_node;
969    struct stable_node *stable_node;
970
971    stable_node = page_stable_node(page);
972    if (stable_node) { /* ksm page forked */
973        get_page(page);
974        return page;
975    }
976
977    while (node) {
978        struct page *tree_page;
979        int ret;
980
981        cond_resched();
982        stable_node = rb_entry(node, struct stable_node, node);
983        tree_page = get_ksm_page(stable_node);
984        if (!tree_page)
985            return NULL;
986
987        ret = memcmp_pages(page, tree_page);
988
989        if (ret < 0) {
990            put_page(tree_page);
991            node = node->rb_left;
992        } else if (ret > 0) {
993            put_page(tree_page);
994            node = node->rb_right;
995        } else
996            return tree_page;
997    }
998
999    return NULL;
1000}
1001
1002/*
1003 * stable_tree_insert - insert rmap_item pointing to new ksm page
1004 * into the stable tree.
1005 *
1006 * This function returns the stable tree node just allocated on success,
1007 * NULL otherwise.
1008 */
1009static struct stable_node *stable_tree_insert(struct page *kpage)
1010{
1011    struct rb_node **new = &root_stable_tree.rb_node;
1012    struct rb_node *parent = NULL;
1013    struct stable_node *stable_node;
1014
1015    while (*new) {
1016        struct page *tree_page;
1017        int ret;
1018
1019        cond_resched();
1020        stable_node = rb_entry(*new, struct stable_node, node);
1021        tree_page = get_ksm_page(stable_node);
1022        if (!tree_page)
1023            return NULL;
1024
1025        ret = memcmp_pages(kpage, tree_page);
1026        put_page(tree_page);
1027
1028        parent = *new;
1029        if (ret < 0)
1030            new = &parent->rb_left;
1031        else if (ret > 0)
1032            new = &parent->rb_right;
1033        else {
1034            /*
1035             * It is not a bug that stable_tree_search() didn't
1036             * find this node: because at that time our page was
1037             * not yet write-protected, so may have changed since.
1038             */
1039            return NULL;
1040        }
1041    }
1042
1043    stable_node = alloc_stable_node();
1044    if (!stable_node)
1045        return NULL;
1046
1047    rb_link_node(&stable_node->node, parent, new);
1048    rb_insert_color(&stable_node->node, &root_stable_tree);
1049
1050    INIT_HLIST_HEAD(&stable_node->hlist);
1051
1052    stable_node->kpfn = page_to_pfn(kpage);
1053    set_page_stable_node(kpage, stable_node);
1054
1055    return stable_node;
1056}
1057
1058/*
1059 * unstable_tree_search_insert - search for identical page,
1060 * else insert rmap_item into the unstable tree.
1061 *
1062 * This function searches for a page in the unstable tree identical to the
1063 * page currently being scanned; and if no identical page is found in the
1064 * tree, we insert rmap_item as a new object into the unstable tree.
1065 *
1066 * This function returns pointer to rmap_item found to be identical
1067 * to the currently scanned page, NULL otherwise.
1068 *
1069 * This function does both searching and inserting, because they share
1070 * the same walking algorithm in an rbtree.
1071 */
1072static
1073struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
1074                          struct page *page,
1075                          struct page **tree_pagep)
1076
1077{
1078    struct rb_node **new = &root_unstable_tree.rb_node;
1079    struct rb_node *parent = NULL;
1080
1081    while (*new) {
1082        struct rmap_item *tree_rmap_item;
1083        struct page *tree_page;
1084        int ret;
1085
1086        cond_resched();
1087        tree_rmap_item = rb_entry(*new, struct rmap_item, node);
1088        tree_page = get_mergeable_page(tree_rmap_item);
1089        if (IS_ERR_OR_NULL(tree_page))
1090            return NULL;
1091
1092        /*
1093         * Don't substitute a ksm page for a forked page.
1094         */
1095        if (page == tree_page) {
1096            put_page(tree_page);
1097            return NULL;
1098        }
1099
1100        ret = memcmp_pages(page, tree_page);
1101
1102        parent = *new;
1103        if (ret < 0) {
1104            put_page(tree_page);
1105            new = &parent->rb_left;
1106        } else if (ret > 0) {
1107            put_page(tree_page);
1108            new = &parent->rb_right;
1109        } else {
1110            *tree_pagep = tree_page;
1111            return tree_rmap_item;
1112        }
1113    }
1114
1115    rmap_item->address |= UNSTABLE_FLAG;
1116    rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
1117    rb_link_node(&rmap_item->node, parent, new);
1118    rb_insert_color(&rmap_item->node, &root_unstable_tree);
1119
1120    ksm_pages_unshared++;
1121    return NULL;
1122}
1123
1124/*
1125 * stable_tree_append - add another rmap_item to the linked list of
1126 * rmap_items hanging off a given node of the stable tree, all sharing
1127 * the same ksm page.
1128 */
1129static void stable_tree_append(struct rmap_item *rmap_item,
1130                   struct stable_node *stable_node)
1131{
1132    rmap_item->head = stable_node;
1133    rmap_item->address |= STABLE_FLAG;
1134    hlist_add_head(&rmap_item->hlist, &stable_node->hlist);
1135
1136    if (rmap_item->hlist.next)
1137        ksm_pages_sharing++;
1138    else
1139        ksm_pages_shared++;
1140}
1141
1142/*
1143 * cmp_and_merge_page - first see if page can be merged into the stable tree;
1144 * if not, compare checksum to previous and if it's the same, see if page can
1145 * be inserted into the unstable tree, or merged with a page already there and
1146 * both transferred to the stable tree.
1147 *
1148 * @page: the page that we are searching identical page to.
1149 * @rmap_item: the reverse mapping into the virtual address of this page
1150 */
1151static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
1152{
1153    struct rmap_item *tree_rmap_item;
1154    struct page *tree_page = NULL;
1155    struct stable_node *stable_node;
1156    struct page *kpage;
1157    unsigned int checksum;
1158    int err;
1159
1160    remove_rmap_item_from_tree(rmap_item);
1161
1162    /* We first start with searching the page inside the stable tree */
1163    kpage = stable_tree_search(page);
1164    if (kpage) {
1165        err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
1166        if (!err) {
1167            /*
1168             * The page was successfully merged:
1169             * add its rmap_item to the stable tree.
1170             */
1171            lock_page(kpage);
1172            stable_tree_append(rmap_item, page_stable_node(kpage));
1173            unlock_page(kpage);
1174        }
1175        put_page(kpage);
1176        return;
1177    }
1178
1179    /*
1180     * If the hash value of the page has changed from the last time
1181     * we calculated it, this page is changing frequently: therefore we
1182     * don't want to insert it in the unstable tree, and we don't want
1183     * to waste our time searching for something identical to it there.
1184     */
1185    checksum = calc_checksum(page);
1186    if (rmap_item->oldchecksum != checksum) {
1187        rmap_item->oldchecksum = checksum;
1188        return;
1189    }
1190
1191    tree_rmap_item =
1192        unstable_tree_search_insert(rmap_item, page, &tree_page);
1193    if (tree_rmap_item) {
1194        kpage = try_to_merge_two_pages(rmap_item, page,
1195                        tree_rmap_item, tree_page);
1196        put_page(tree_page);
1197        /*
1198         * As soon as we merge this page, we want to remove the
1199         * rmap_item of the page we have merged with from the unstable
1200         * tree, and insert it instead as new node in the stable tree.
1201         */
1202        if (kpage) {
1203            remove_rmap_item_from_tree(tree_rmap_item);
1204
1205            lock_page(kpage);
1206            stable_node = stable_tree_insert(kpage);
1207            if (stable_node) {
1208                stable_tree_append(tree_rmap_item, stable_node);
1209                stable_tree_append(rmap_item, stable_node);
1210            }
1211            unlock_page(kpage);
1212
1213            /*
1214             * If we fail to insert the page into the stable tree,
1215             * we will have 2 virtual addresses that are pointing
1216             * to a ksm page left outside the stable tree,
1217             * in which case we need to break_cow on both.
1218             */
1219            if (!stable_node) {
1220                break_cow(tree_rmap_item);
1221                break_cow(rmap_item);
1222            }
1223        }
1224    }
1225}
1226
1227static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
1228                        struct rmap_item **rmap_list,
1229                        unsigned long addr)
1230{
1231    struct rmap_item *rmap_item;
1232
1233    while (*rmap_list) {
1234        rmap_item = *rmap_list;
1235        if ((rmap_item->address & PAGE_MASK) == addr)
1236            return rmap_item;
1237        if (rmap_item->address > addr)
1238            break;
1239        *rmap_list = rmap_item->rmap_list;
1240        remove_rmap_item_from_tree(rmap_item);
1241        free_rmap_item(rmap_item);
1242    }
1243
1244    rmap_item = alloc_rmap_item();
1245    if (rmap_item) {
1246        /* It has already been zeroed */
1247        rmap_item->mm = mm_slot->mm;
1248        rmap_item->address = addr;
1249        rmap_item->rmap_list = *rmap_list;
1250        *rmap_list = rmap_item;
1251    }
1252    return rmap_item;
1253}
1254
1255static struct rmap_item *scan_get_next_rmap_item(struct page **page)
1256{
1257    struct mm_struct *mm;
1258    struct mm_slot *slot;
1259    struct vm_area_struct *vma;
1260    struct rmap_item *rmap_item;
1261
1262    if (list_empty(&ksm_mm_head.mm_list))
1263        return NULL;
1264
1265    slot = ksm_scan.mm_slot;
1266    if (slot == &ksm_mm_head) {
1267        root_unstable_tree = RB_ROOT;
1268
1269        spin_lock(&ksm_mmlist_lock);
1270        slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1271        ksm_scan.mm_slot = slot;
1272        spin_unlock(&ksm_mmlist_lock);
1273next_mm:
1274        ksm_scan.address = 0;
1275        ksm_scan.rmap_list = &slot->rmap_list;
1276    }
1277
1278    mm = slot->mm;
1279    down_read(&mm->mmap_sem);
1280    if (ksm_test_exit(mm))
1281        vma = NULL;
1282    else
1283        vma = find_vma(mm, ksm_scan.address);
1284
1285    for (; vma; vma = vma->vm_next) {
1286        if (!(vma->vm_flags & VM_MERGEABLE))
1287            continue;
1288        if (ksm_scan.address < vma->vm_start)
1289            ksm_scan.address = vma->vm_start;
1290        if (!vma->anon_vma)
1291            ksm_scan.address = vma->vm_end;
1292
1293        while (ksm_scan.address < vma->vm_end) {
1294            if (ksm_test_exit(mm))
1295                break;
1296            *page = follow_page(vma, ksm_scan.address, FOLL_GET);
1297            if (!IS_ERR_OR_NULL(*page) && PageAnon(*page)) {
1298                flush_anon_page(vma, *page, ksm_scan.address);
1299                flush_dcache_page(*page);
1300                rmap_item = get_next_rmap_item(slot,
1301                    ksm_scan.rmap_list, ksm_scan.address);
1302                if (rmap_item) {
1303                    ksm_scan.rmap_list =
1304                            &rmap_item->rmap_list;
1305                    ksm_scan.address += PAGE_SIZE;
1306                } else
1307                    put_page(*page);
1308                up_read(&mm->mmap_sem);
1309                return rmap_item;
1310            }
1311            if (!IS_ERR_OR_NULL(*page))
1312                put_page(*page);
1313            ksm_scan.address += PAGE_SIZE;
1314            cond_resched();
1315        }
1316    }
1317
1318    if (ksm_test_exit(mm)) {
1319        ksm_scan.address = 0;
1320        ksm_scan.rmap_list = &slot->rmap_list;
1321    }
1322    /*
1323     * Nuke all the rmap_items that are above this current rmap:
1324     * because there were no VM_MERGEABLE vmas with such addresses.
1325     */
1326    remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
1327
1328    spin_lock(&ksm_mmlist_lock);
1329    ksm_scan.mm_slot = list_entry(slot->mm_list.next,
1330                        struct mm_slot, mm_list);
1331    if (ksm_scan.address == 0) {
1332        /*
1333         * We've completed a full scan of all vmas, holding mmap_sem
1334         * throughout, and found no VM_MERGEABLE: so do the same as
1335         * __ksm_exit does to remove this mm from all our lists now.
1336         * This applies either when cleaning up after __ksm_exit
1337         * (but beware: we can reach here even before __ksm_exit),
1338         * or when all VM_MERGEABLE areas have been unmapped (and
1339         * mmap_sem then protects against race with MADV_MERGEABLE).
1340         */
1341        hlist_del(&slot->link);
1342        list_del(&slot->mm_list);
1343        spin_unlock(&ksm_mmlist_lock);
1344
1345        free_mm_slot(slot);
1346        clear_bit(MMF_VM_MERGEABLE, &mm->flags);
1347        up_read(&mm->mmap_sem);
1348        mmdrop(mm);
1349    } else {
1350        spin_unlock(&ksm_mmlist_lock);
1351        up_read(&mm->mmap_sem);
1352    }
1353
1354    /* Repeat until we've completed scanning the whole list */
1355    slot = ksm_scan.mm_slot;
1356    if (slot != &ksm_mm_head)
1357        goto next_mm;
1358
1359    ksm_scan.seqnr++;
1360    return NULL;
1361}
1362
1363/**
1364 * ksm_do_scan - the ksm scanner main worker function.
1365 * @scan_npages - number of pages we want to scan before we return.
1366 */
1367static void ksm_do_scan(unsigned int scan_npages)
1368{
1369    struct rmap_item *rmap_item;
1370    struct page *uninitialized_var(page);
1371
1372    while (scan_npages--) {
1373        cond_resched();
1374        rmap_item = scan_get_next_rmap_item(&page);
1375        if (!rmap_item)
1376            return;
1377        if (!PageKsm(page) || !in_stable_tree(rmap_item))
1378            cmp_and_merge_page(page, rmap_item);
1379        put_page(page);
1380    }
1381}
1382
1383static int ksmd_should_run(void)
1384{
1385    return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
1386}
1387
1388static int ksm_scan_thread(void *nothing)
1389{
1390    set_user_nice(current, 5);
1391
1392    while (!kthread_should_stop()) {
1393        mutex_lock(&ksm_thread_mutex);
1394        if (ksmd_should_run())
1395            ksm_do_scan(ksm_thread_pages_to_scan);
1396        mutex_unlock(&ksm_thread_mutex);
1397
1398        if (ksmd_should_run()) {
1399            schedule_timeout_interruptible(
1400                msecs_to_jiffies(ksm_thread_sleep_millisecs));
1401        } else {
1402            wait_event_interruptible(ksm_thread_wait,
1403                ksmd_should_run() || kthread_should_stop());
1404        }
1405    }
1406    return 0;
1407}
1408
1409int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
1410        unsigned long end, int advice, unsigned long *vm_flags)
1411{
1412    struct mm_struct *mm = vma->vm_mm;
1413    int err;
1414
1415    switch (advice) {
1416    case MADV_MERGEABLE:
1417        /*
1418         * Be somewhat over-protective for now!
1419         */
1420        if (*vm_flags & (VM_MERGEABLE | VM_SHARED | VM_MAYSHARE |
1421                 VM_PFNMAP | VM_IO | VM_DONTEXPAND |
1422                 VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE |
1423                 VM_NONLINEAR | VM_MIXEDMAP | VM_SAO))
1424            return 0; /* just ignore the advice */
1425
1426        if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
1427            err = __ksm_enter(mm);
1428            if (err)
1429                return err;
1430        }
1431
1432        *vm_flags |= VM_MERGEABLE;
1433        break;
1434
1435    case MADV_UNMERGEABLE:
1436        if (!(*vm_flags & VM_MERGEABLE))
1437            return 0; /* just ignore the advice */
1438
1439        if (vma->anon_vma) {
1440            err = unmerge_ksm_pages(vma, start, end);
1441            if (err)
1442                return err;
1443        }
1444
1445        *vm_flags &= ~VM_MERGEABLE;
1446        break;
1447    }
1448
1449    return 0;
1450}
1451
1452int __ksm_enter(struct mm_struct *mm)
1453{
1454    struct mm_slot *mm_slot;
1455    int needs_wakeup;
1456
1457    mm_slot = alloc_mm_slot();
1458    if (!mm_slot)
1459        return -ENOMEM;
1460
1461    /* Check ksm_run too? Would need tighter locking */
1462    needs_wakeup = list_empty(&ksm_mm_head.mm_list);
1463
1464    spin_lock(&ksm_mmlist_lock);
1465    insert_to_mm_slots_hash(mm, mm_slot);
1466    /*
1467     * Insert just behind the scanning cursor, to let the area settle
1468     * down a little; when fork is followed by immediate exec, we don't
1469     * want ksmd to waste time setting up and tearing down an rmap_list.
1470     */
1471    list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
1472    spin_unlock(&ksm_mmlist_lock);
1473
1474    set_bit(MMF_VM_MERGEABLE, &mm->flags);
1475    atomic_inc(&mm->mm_count);
1476
1477    if (needs_wakeup)
1478        wake_up_interruptible(&ksm_thread_wait);
1479
1480    return 0;
1481}
1482
1483void __ksm_exit(struct mm_struct *mm)
1484{
1485    struct mm_slot *mm_slot;
1486    int easy_to_free = 0;
1487
1488    /*
1489     * This process is exiting: if it's straightforward (as is the
1490     * case when ksmd was never running), free mm_slot immediately.
1491     * But if it's at the cursor or has rmap_items linked to it, use
1492     * mmap_sem to synchronize with any break_cows before pagetables
1493     * are freed, and leave the mm_slot on the list for ksmd to free.
1494     * Beware: ksm may already have noticed it exiting and freed the slot.
1495     */
1496
1497    spin_lock(&ksm_mmlist_lock);
1498    mm_slot = get_mm_slot(mm);
1499    if (mm_slot && ksm_scan.mm_slot != mm_slot) {
1500        if (!mm_slot->rmap_list) {
1501            hlist_del(&mm_slot->link);
1502            list_del(&mm_slot->mm_list);
1503            easy_to_free = 1;
1504        } else {
1505            list_move(&mm_slot->mm_list,
1506                  &ksm_scan.mm_slot->mm_list);
1507        }
1508    }
1509    spin_unlock(&ksm_mmlist_lock);
1510
1511    if (easy_to_free) {
1512        free_mm_slot(mm_slot);
1513        clear_bit(MMF_VM_MERGEABLE, &mm->flags);
1514        mmdrop(mm);
1515    } else if (mm_slot) {
1516        down_write(&mm->mmap_sem);
1517        up_write(&mm->mmap_sem);
1518    }
1519}
1520
1521struct page *ksm_does_need_to_copy(struct page *page,
1522            struct vm_area_struct *vma, unsigned long address)
1523{
1524    struct page *new_page;
1525
1526    unlock_page(page); /* any racers will COW it, not modify it */
1527
1528    new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
1529    if (new_page) {
1530        copy_user_highpage(new_page, page, address, vma);
1531
1532        SetPageDirty(new_page);
1533        __SetPageUptodate(new_page);
1534        SetPageSwapBacked(new_page);
1535        __set_page_locked(new_page);
1536
1537        if (page_evictable(new_page, vma))
1538            lru_cache_add_lru(new_page, LRU_ACTIVE_ANON);
1539        else
1540            add_page_to_unevictable_list(new_page);
1541    }
1542
1543    page_cache_release(page);
1544    return new_page;
1545}
1546
1547int page_referenced_ksm(struct page *page, struct mem_cgroup *memcg,
1548            unsigned long *vm_flags)
1549{
1550    struct stable_node *stable_node;
1551    struct rmap_item *rmap_item;
1552    struct hlist_node *hlist;
1553    unsigned int mapcount = page_mapcount(page);
1554    int referenced = 0;
1555    int search_new_forks = 0;
1556
1557    VM_BUG_ON(!PageKsm(page));
1558    VM_BUG_ON(!PageLocked(page));
1559
1560    stable_node = page_stable_node(page);
1561    if (!stable_node)
1562        return 0;
1563again:
1564    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
1565        struct anon_vma *anon_vma = rmap_item->anon_vma;
1566        struct anon_vma_chain *vmac;
1567        struct vm_area_struct *vma;
1568
1569        spin_lock(&anon_vma->lock);
1570        list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
1571            vma = vmac->vma;
1572            if (rmap_item->address < vma->vm_start ||
1573                rmap_item->address >= vma->vm_end)
1574                continue;
1575            /*
1576             * Initially we examine only the vma which covers this
1577             * rmap_item; but later, if there is still work to do,
1578             * we examine covering vmas in other mms: in case they
1579             * were forked from the original since ksmd passed.
1580             */
1581            if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
1582                continue;
1583
1584            if (memcg && !mm_match_cgroup(vma->vm_mm, memcg))
1585                continue;
1586
1587            referenced += page_referenced_one(page, vma,
1588                rmap_item->address, &mapcount, vm_flags);
1589            if (!search_new_forks || !mapcount)
1590                break;
1591        }
1592        spin_unlock(&anon_vma->lock);
1593        if (!mapcount)
1594            goto out;
1595    }
1596    if (!search_new_forks++)
1597        goto again;
1598out:
1599    return referenced;
1600}
1601
1602int try_to_unmap_ksm(struct page *page, enum ttu_flags flags)
1603{
1604    struct stable_node *stable_node;
1605    struct hlist_node *hlist;
1606    struct rmap_item *rmap_item;
1607    int ret = SWAP_AGAIN;
1608    int search_new_forks = 0;
1609
1610    VM_BUG_ON(!PageKsm(page));
1611    VM_BUG_ON(!PageLocked(page));
1612
1613    stable_node = page_stable_node(page);
1614    if (!stable_node)
1615        return SWAP_FAIL;
1616again:
1617    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
1618        struct anon_vma *anon_vma = rmap_item->anon_vma;
1619        struct anon_vma_chain *vmac;
1620        struct vm_area_struct *vma;
1621
1622        spin_lock(&anon_vma->lock);
1623        list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
1624            vma = vmac->vma;
1625            if (rmap_item->address < vma->vm_start ||
1626                rmap_item->address >= vma->vm_end)
1627                continue;
1628            /*
1629             * Initially we examine only the vma which covers this
1630             * rmap_item; but later, if there is still work to do,
1631             * we examine covering vmas in other mms: in case they
1632             * were forked from the original since ksmd passed.
1633             */
1634            if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
1635                continue;
1636
1637            ret = try_to_unmap_one(page, vma,
1638                    rmap_item->address, flags);
1639            if (ret != SWAP_AGAIN || !page_mapped(page)) {
1640                spin_unlock(&anon_vma->lock);
1641                goto out;
1642            }
1643        }
1644        spin_unlock(&anon_vma->lock);
1645    }
1646    if (!search_new_forks++)
1647        goto again;
1648out:
1649    return ret;
1650}
1651
1652#ifdef CONFIG_MIGRATION
1653int rmap_walk_ksm(struct page *page, int (*rmap_one)(struct page *,
1654          struct vm_area_struct *, unsigned long, void *), void *arg)
1655{
1656    struct stable_node *stable_node;
1657    struct hlist_node *hlist;
1658    struct rmap_item *rmap_item;
1659    int ret = SWAP_AGAIN;
1660    int search_new_forks = 0;
1661
1662    VM_BUG_ON(!PageKsm(page));
1663    VM_BUG_ON(!PageLocked(page));
1664
1665    stable_node = page_stable_node(page);
1666    if (!stable_node)
1667        return ret;
1668again:
1669    hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
1670        struct anon_vma *anon_vma = rmap_item->anon_vma;
1671        struct anon_vma_chain *vmac;
1672        struct vm_area_struct *vma;
1673
1674        spin_lock(&anon_vma->lock);
1675        list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
1676            vma = vmac->vma;
1677            if (rmap_item->address < vma->vm_start ||
1678                rmap_item->address >= vma->vm_end)
1679                continue;
1680            /*
1681             * Initially we examine only the vma which covers this
1682             * rmap_item; but later, if there is still work to do,
1683             * we examine covering vmas in other mms: in case they
1684             * were forked from the original since ksmd passed.
1685             */
1686            if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
1687                continue;
1688
1689            ret = rmap_one(page, vma, rmap_item->address, arg);
1690            if (ret != SWAP_AGAIN) {
1691                spin_unlock(&anon_vma->lock);
1692                goto out;
1693            }
1694        }
1695        spin_unlock(&anon_vma->lock);
1696    }
1697    if (!search_new_forks++)
1698        goto again;
1699out:
1700    return ret;
1701}
1702
1703void ksm_migrate_page(struct page *newpage, struct page *oldpage)
1704{
1705    struct stable_node *stable_node;
1706
1707    VM_BUG_ON(!PageLocked(oldpage));
1708    VM_BUG_ON(!PageLocked(newpage));
1709    VM_BUG_ON(newpage->mapping != oldpage->mapping);
1710
1711    stable_node = page_stable_node(newpage);
1712    if (stable_node) {
1713        VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage));
1714        stable_node->kpfn = page_to_pfn(newpage);
1715    }
1716}
1717#endif /* CONFIG_MIGRATION */
1718
1719#ifdef CONFIG_MEMORY_HOTREMOVE
1720static struct stable_node *ksm_check_stable_tree(unsigned long start_pfn,
1721                         unsigned long end_pfn)
1722{
1723    struct rb_node *node;
1724
1725    for (node = rb_first(&root_stable_tree); node; node = rb_next(node)) {
1726        struct stable_node *stable_node;
1727
1728        stable_node = rb_entry(node, struct stable_node, node);
1729        if (stable_node->kpfn >= start_pfn &&
1730            stable_node->kpfn < end_pfn)
1731            return stable_node;
1732    }
1733    return NULL;
1734}
1735
1736static int ksm_memory_callback(struct notifier_block *self,
1737                   unsigned long action, void *arg)
1738{
1739    struct memory_notify *mn = arg;
1740    struct stable_node *stable_node;
1741
1742    switch (action) {
1743    case MEM_GOING_OFFLINE:
1744        /*
1745         * Keep it very simple for now: just lock out ksmd and
1746         * MADV_UNMERGEABLE while any memory is going offline.
1747         */
1748        mutex_lock(&ksm_thread_mutex);
1749        break;
1750
1751    case MEM_OFFLINE:
1752        /*
1753         * Most of the work is done by page migration; but there might
1754         * be a few stable_nodes left over, still pointing to struct
1755         * pages which have been offlined: prune those from the tree.
1756         */
1757        while ((stable_node = ksm_check_stable_tree(mn->start_pfn,
1758                    mn->start_pfn + mn->nr_pages)) != NULL)
1759            remove_node_from_stable_tree(stable_node);
1760        /* fallthrough */
1761
1762    case MEM_CANCEL_OFFLINE:
1763        mutex_unlock(&ksm_thread_mutex);
1764        break;
1765    }
1766    return NOTIFY_OK;
1767}
1768#endif /* CONFIG_MEMORY_HOTREMOVE */
1769
1770#ifdef CONFIG_SYSFS
1771/*
1772 * This all compiles without CONFIG_SYSFS, but is a waste of space.
1773 */
1774
1775#define KSM_ATTR_RO(_name) \
1776    static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
1777#define KSM_ATTR(_name) \
1778    static struct kobj_attribute _name##_attr = \
1779        __ATTR(_name, 0644, _name##_show, _name##_store)
1780
1781static ssize_t sleep_millisecs_show(struct kobject *kobj,
1782                    struct kobj_attribute *attr, char *buf)
1783{
1784    return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
1785}
1786
1787static ssize_t sleep_millisecs_store(struct kobject *kobj,
1788                     struct kobj_attribute *attr,
1789                     const char *buf, size_t count)
1790{
1791    unsigned long msecs;
1792    int err;
1793
1794    err = strict_strtoul(buf, 10, &msecs);
1795    if (err || msecs > UINT_MAX)
1796        return -EINVAL;
1797
1798    ksm_thread_sleep_millisecs = msecs;
1799
1800    return count;
1801}
1802KSM_ATTR(sleep_millisecs);
1803
1804static ssize_t pages_to_scan_show(struct kobject *kobj,
1805                  struct kobj_attribute *attr, char *buf)
1806{
1807    return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
1808}
1809
1810static ssize_t pages_to_scan_store(struct kobject *kobj,
1811                   struct kobj_attribute *attr,
1812                   const char *buf, size_t count)
1813{
1814    int err;
1815    unsigned long nr_pages;
1816
1817    err = strict_strtoul(buf, 10, &nr_pages);
1818    if (err || nr_pages > UINT_MAX)
1819        return -EINVAL;
1820
1821    ksm_thread_pages_to_scan = nr_pages;
1822
1823    return count;
1824}
1825KSM_ATTR(pages_to_scan);
1826
1827static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
1828            char *buf)
1829{
1830    return sprintf(buf, "%u\n", ksm_run);
1831}
1832
1833static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
1834             const char *buf, size_t count)
1835{
1836    int err;
1837    unsigned long flags;
1838
1839    err = strict_strtoul(buf, 10, &flags);
1840    if (err || flags > UINT_MAX)
1841        return -EINVAL;
1842    if (flags > KSM_RUN_UNMERGE)
1843        return -EINVAL;
1844
1845    /*
1846     * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
1847     * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
1848     * breaking COW to free the pages_shared (but leaves mm_slots
1849     * on the list for when ksmd may be set running again).
1850     */
1851
1852    mutex_lock(&ksm_thread_mutex);
1853    if (ksm_run != flags) {
1854        ksm_run = flags;
1855        if (flags & KSM_RUN_UNMERGE) {
1856            current->flags |= PF_OOM_ORIGIN;
1857            err = unmerge_and_remove_all_rmap_items();
1858            current->flags &= ~PF_OOM_ORIGIN;
1859            if (err) {
1860                ksm_run = KSM_RUN_STOP;
1861                count = err;
1862            }
1863        }
1864    }
1865    mutex_unlock(&ksm_thread_mutex);
1866
1867    if (flags & KSM_RUN_MERGE)
1868        wake_up_interruptible(&ksm_thread_wait);
1869
1870    return count;
1871}
1872KSM_ATTR(run);
1873
1874static ssize_t pages_shared_show(struct kobject *kobj,
1875                 struct kobj_attribute *attr, char *buf)
1876{
1877    return sprintf(buf, "%lu\n", ksm_pages_shared);
1878}
1879KSM_ATTR_RO(pages_shared);
1880
1881static ssize_t pages_sharing_show(struct kobject *kobj,
1882                  struct kobj_attribute *attr, char *buf)
1883{
1884    return sprintf(buf, "%lu\n", ksm_pages_sharing);
1885}
1886KSM_ATTR_RO(pages_sharing);
1887
1888static ssize_t pages_unshared_show(struct kobject *kobj,
1889                   struct kobj_attribute *attr, char *buf)
1890{
1891    return sprintf(buf, "%lu\n", ksm_pages_unshared);
1892}
1893KSM_ATTR_RO(pages_unshared);
1894
1895static ssize_t pages_volatile_show(struct kobject *kobj,
1896                   struct kobj_attribute *attr, char *buf)
1897{
1898    long ksm_pages_volatile;
1899
1900    ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
1901                - ksm_pages_sharing - ksm_pages_unshared;
1902    /*
1903     * It was not worth any locking to calculate that statistic,
1904     * but it might therefore sometimes be negative: conceal that.
1905     */
1906    if (ksm_pages_volatile < 0)
1907        ksm_pages_volatile = 0;
1908    return sprintf(buf, "%ld\n", ksm_pages_volatile);
1909}
1910KSM_ATTR_RO(pages_volatile);
1911
1912static ssize_t full_scans_show(struct kobject *kobj,
1913                   struct kobj_attribute *attr, char *buf)
1914{
1915    return sprintf(buf, "%lu\n", ksm_scan.seqnr);
1916}
1917KSM_ATTR_RO(full_scans);
1918
1919static struct attribute *ksm_attrs[] = {
1920    &sleep_millisecs_attr.attr,
1921    &pages_to_scan_attr.attr,
1922    &run_attr.attr,
1923    &pages_shared_attr.attr,
1924    &pages_sharing_attr.attr,
1925    &pages_unshared_attr.attr,
1926    &pages_volatile_attr.attr,
1927    &full_scans_attr.attr,
1928    NULL,
1929};
1930
1931static struct attribute_group ksm_attr_group = {
1932    .attrs = ksm_attrs,
1933    .name = "ksm",
1934};
1935#endif /* CONFIG_SYSFS */
1936
1937static int __init ksm_init(void)
1938{
1939    struct task_struct *ksm_thread;
1940    int err;
1941
1942    err = ksm_slab_init();
1943    if (err)
1944        goto out;
1945
1946    err = mm_slots_hash_init();
1947    if (err)
1948        goto out_free1;
1949
1950    ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
1951    if (IS_ERR(ksm_thread)) {
1952        printk(KERN_ERR "ksm: creating kthread failed\n");
1953        err = PTR_ERR(ksm_thread);
1954        goto out_free2;
1955    }
1956
1957#ifdef CONFIG_SYSFS
1958    err = sysfs_create_group(mm_kobj, &ksm_attr_group);
1959    if (err) {
1960        printk(KERN_ERR "ksm: register sysfs failed\n");
1961        kthread_stop(ksm_thread);
1962        goto out_free2;
1963    }
1964#else
1965    ksm_run = KSM_RUN_MERGE; /* no way for user to start it */
1966
1967#endif /* CONFIG_SYSFS */
1968
1969#ifdef CONFIG_MEMORY_HOTREMOVE
1970    /*
1971     * Choose a high priority since the callback takes ksm_thread_mutex:
1972     * later callbacks could only be taking locks which nest within that.
1973     */
1974    hotplug_memory_notifier(ksm_memory_callback, 100);
1975#endif
1976    return 0;
1977
1978out_free2:
1979    mm_slots_hash_free();
1980out_free1:
1981    ksm_slab_free();
1982out:
1983    return err;
1984}
1985module_init(ksm_init)
1986

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